The star indexes are associated with the tool RNA STAR and are a requirement/dependency for rMATS.
The rMATS tool is not wrapped for Galaxy (at least in the Main Tool Shed http://usegalaxy.org/toolshed). So, I am assuming that you are running this line command.
For help with install, config, and usage, please see the support resources at that tool's development site: http://rnaseq-mats.sourceforge.net/. Or, consider a general bioinformatics forum such as: https://www.biostars.org/.
Thanks! Jen, Galaxy team